1. Field of the Invention
The present invention relates to locators for locating anomalies in a casing string for a wellbore such as casing joints. More particularly, the invention relates to apparatus and methods for detecting, identifying, and locating anomalies in strings of tubular members by sensing the natural magnetic fields induced within the string, such as perturbations in the natural magnetic fields due to fringe effects caused by the anomalies.
2. Description of the Related Art
Casing collar locators are used to locate joints within the borehole casing. The locator is suspended on a wireline cable and passed through the cased borehole. The locator detects the collars used at joints in the casing string as the locator is moved upwardly and/or downwardly through the casing. Various types of casing joints are used to connect adjacent ends of the casing section in a threaded engagement, such as upset joints and exterior collar joints. As the locator moves adjacent to a casing joint, it detects a change in the magnetic readings resulting from the change in casing thickness, or change in mass of metal associated with the casing wall or it detects a change in the polarity of adjacent sections of casing.
Casing collar locators are extremely important tools for downhole operations. They are required for depth correction operations and for the accurate placement of downhole tools, such as anchors, bridges, whipstocks, profiles, and packers. For example, it is desired to avoid setting a downhole tool on a casing joint since the joint presents a gap or discontinuity in the casing wall that may prevent the downhole tool from sealing or anchoring properly.
In order to detect a casing joint, conventional casing collar locators typically rely on the generation of a relatively powerful magnetic field from the locator using either a permanent magnet or by passing a current through a coil to induce magnetism. A significant amount of power is required to generate the magnetic field. As the coil passes adjacent a casing joint, the flux density of the magnetic field is changed by the variation in the thickness of metal provided by the joint. The change causes an electrical output signal to be generated that indicates the presence of the casing joint, and this output signal is transmitted to the surface of the well through a wireline.
Unfortunately, conventional casing collar locators suffer from operational disadvantages and limitations of their effectiveness. Conventional locators are not greatly sensitive, in general, to discontinuities, anomalies, or other changes in the wall of the casing because prior art locators are necessarily large and often several inches to a few feet in length. This causes the locators to have a large resolution such that they cannot detect changes in the magnetic fields of the casing that are less in length than the locator. Thus, such prior art locators are insensitive to small anomalies in the casing.
As a result of not having a high resolution, conventional casing collar locators are reliable only in a “dynamic” mode wherein the locator is moved rapidly through the wellbore casing in order to accurately detect the presence of casing joints. If the locator is moved too slowly, the changes in the signal indicative of the presence of a casing joint, such as a collar, may be too gradual to be recognized by the well operator. Dynamic location of casing joints thus is disadvantageous because it tends to provide less accurate real-time information concerning the position of the casing joint. For example, if it is desired to set a packer five feet below a particular casing joint in a wellbore, a conventional casing collar locator would be moved rapidly either upwardly or downwardly through the wellbore until the particular casing joint is detected. When that occurs, a signal is provided to the wellbore operator which indicates the location of the joint. Due to movement of the locator through the casing, however, the casing collar locator is no longer positioned proximate the casing joint by the time the operator receives the signal and reacts to it by stopping movement of the locator. The precise position of the casing joint must then be somewhat approximated given the current position of the locator within the wellbore.
Additionally, conventional locators locate casing joints by detecting a difference in thickness of the casing wall such as the presence of an external upset or collar. These devices are actually, “collar” locators rather than “joint” locators. As a result, they are unable to reliably detect a “flush” joint where the casing wall thickness is not appreciably altered by the presence of the joint. A joint is considered flush where the adjacent casing sections are threaded directly to one another or where the upset or collar is unusually thin or contains very little metal.
In addition, because conventional casing collar locators generate a significant magnetic field, they tend to interfere with other downhole instrumentation that rely upon accurate magnetic readings. For example, a compass-type magnetometer that is attempting to find magnetic north can be confused by the magnetic field generated by the casing collar locator. Some induction-type locators are known that generate and transmit strong electromagnetic waves, rather than magnetic fields, to detect casing joints. Unfortunately, these devices also tend to interfere with downhole instrumentation.
A need exists for a locator that can more reliably detect the presence of casing section joints in a wellbore and particularly flush joints that do not employ radially enlarged upsets or collars. Further, a need exists for a locator that generates a minimal or no magnetic field that affects the operation of other downhole instrumentation.
In addition, a need exists for a detector that can detect, identify, and/or locate anomalies, such as deformities, discontinuities, perforations and the like, in a cased borehole. To locate the depth and angular orientation of a perforation, for example, requires a very sensitive locator because of the small size of the perforation. The perforation generally is less than one inch in diameter and typically only one-fourth inch in diameter, thus providing a very small change in the continuity of the casing wall and requiring a very sensitive locator.
By way of background, to complete a well, the cased borehole is perforated adjacent the formation to be produced. A perforating trip is made by lowering into the well bore a perforation tool mounted on the lower end of a wireline or tubular work string. The perforation tool or “gun” assembly is then detonated to create a series of spaced perforations extending outwardly through the well casing, the cement holding the casing in place in the wellbore, and into the production zone. Although these perforations may have a random pattern, typically the perforations are made in a spiral pattern around the casing string.
Often the well is treated to enhance production. Well treatment may include treating the formation with chemicals, “fracturing” or a “fracing” the formation, injection of high pressure fluids, acidizing, jetting, or pumping proppant into the formation to maintain the fractures in the formation. The well is treated or stimulated by pumping fluids through the perforations and into the formation. For example, during fracing, a tubular discharge member having a series of spaced discharge ports is lowered into the well on a work string. Packers are set above and below the perforations to form an isolated region. The discharge ports are preferably aligned with the perforations. A slurry is then pumped down the workstring and discharged through the ports in the discharge member causing the slurry to flow through the perforations and into the surrounding production zone. The slurry may include proppant or other treatment fluid.
Well treatment techniques have several well known problems, limitations, and disadvantages. For example, when the discharge member is lowered into the well bore, it is difficult to obtain a precise alignment (in both the axial and angular directions) between the discharge ports in the discharge member and the perforations in the casing. The usual result is that some degree of misalignment exists between the discharge ports and the perforations. When the ports and perforations are not in alignment, the high pressure fluid must follow a tortuous path before entering the perforations after it is discharged from the discharge member. Because the treatment fluid is discharged at a very high pressure and often is highly abrasive, this tortuous flow path can cause severe abrasion and wear problems in the casing.
In addition, it is important that the packer or packers not be set in the perforated region of the casing. If a packer is set in the area having the perforations, the fluid flowing out of discharge ports and through the perforations into the formation may flow back into the wellbore annulus through perforations that are above or below that portion of the wellbore annulus that is isolated by the packers. Turbulence caused by the high pressure and abrasive fluid flowing back into the annulus creates a pressure differential across the packers and tends to erode or “wash out” and ruin the packers. Additionally, it is important that the packer or packers not be set within a casing joint, but instead be set in blank pipe. Typically, there are gaps between the aligned ends of casing sections at the casing joints. If the packer is set in this region, then the packer will not seal properly and hold pressure to isolate the intended interval. When this occurs, the treatment fluid can pass out of the interval and into the annulus and wash out and erode the packer. Accordingly, it is critical to know the location of the perforations and the casing joints to ensure that the packer is not set within the perforations or within a casing joint. Unfortunately, properly positioning the packer with respect to the perforations and casing joints has been difficult to achieve.
Furthermore, even if the depth of the discharge member is precisely known, there still exist problems that are introduced due to inaccuracies in determining the actual depth of the perforations. As stated above, the step of perforating the well typically includes recording the depth and location of the perforations; however, using perforation equipment with both wireline and tubing nevertheless does not always provide accurate depth measurements, due again to the tendency of the tubing or wireline to expand with down hole temperatures or to bend in the borehole.
A need thus exists for a detector that can more reliably detect the presence of anomalies, such as perforations, in the cased borehole. Further, a need exists for a detector that generates a minimal or no magnetic field that would affect the operation of other downhole instrumentation.
Giant magnetoresistive or GMR magnetic field sensors are know for use in high accuracy compasses and geophysical applications such as magnetic field anomaly detection in the earth's crust. GMR sensors are constructed from alternating, ultrathin layers of magnetic and non-magnetic materials. GMR sensors provide high sensitivity to changes in a nearby or surrounding magnetic field. GMR sensors of this type are described in the prior art NVE brochure entitled “NVE—Nonvolatile Electronics, Inc. The GMR Specialists” with errata sheets, and are currently manufactured and marketed by Nonvolatile Electronics, Inc., 11409 Valley View Road, Eden Prairie, Minn. 55344-3617, (612) 829-9217. The GMR sensor uses a “giant magnetoresistive effect” to detect a change in electrical resistance that occurs when stacked layers of ferromagnetic and non-magnetic materials are exposed to a magnetic field.
The present invention overcomes the deficiencies in the prior art.